1. Field of the Invention
The present invention generally relates to a circuit breaker and a circuit braking apparatus using the same, both of which are adapted to be employed in a distribution system. More particularly, the present invention is concerned with a circuit breaker and a circuit breaking apparatus equipped with a current limiting function which are advantageously suited for static implementation of a circuit breaker employed for transmitting out an electric power in a power distribution system.
2. Description of Related Art
For better understanding of the present invention, background techniques thereof will first be reviewed in some detail. FIG. 7 is a circuit diagram showing a static type switch for a distribution system which is disclosed in Transactions of the 10-th National Convention of The Institute of Electrical Engineers of Japan: "Power Protection 10-106/107" (1988) and which typifies a known circuit breaker implemented by using thyristors.
Referring to the figure, a main circuit MC is composed of input terminals U1, V1 and W1 which correspond to U-, V- and W-phases of a distribution system, respectively, output terminals U2, V2 and W2 corresponding to the U-, V- and W-phases, respectively, and thyristor switches 1U, 1V and 1W of non-self-extinction type each constituted by a pair of thyristors connected in anti-parallel to each other. Each of the thyristors constituting the non-self-extinction type thyristor switches 1U, 1V and 1W is of such nature that once the thyristor is turned on, it is required for turning off the same to apply a backward voltage between the anode and the cathode thereof or alternatively to lower the forward current below a holding current. For this reason, the thyristor switch constituted by the thyristors mentioned above is referred to as the non-self-extinction type thyristor switch.
Further referring to FIG. 7, one end of the thyristor switch 1U is connected to the input terminal U1 by way of a vacuum electromagnetic contactor 2U while the other end of the thyristor switch 1U is connected to the output terminal U2. Similarly, the thyristor switch 1V has one end connected to the input terminal V1 via a vacuum electromagnetic contactor 2V with the other end of the thyristor switch 1V being connected to the output terminal V2. On the other hand, the thyristor switch 1W has one end connected to the input terminal W1 via a vacuum electromagnetic contactor 2W with the other end being connected to the output terminal W2.
Furthermore, zinc oxide type arresters 3U, 3V and 3W are connected in parallel to the thyristor switches 1U, 1V and 1W, respectively, for the purpose of protecting these switches against a surge voltage. Current transformers 4a and 4b serving as current detecting means are connected between the thyristor switch 1U and the output terminal U2 as well as between the thyristor switch 1W and the output terminal W2 for detecting the conducting currents, respectively. A control unit 5 which operates at a lower potential level than the main circuit MC serves for controlling the opening/closing of the vacuum electromagnetic contactors 2U, 2V and 2W as well as the turn-on/off of the thyristor switches 1U, 1V and 1W on the basis of the detected currents supplied from the current transformers 4a and 4b.
Next, description will turn to the operation of the switch apparatus described above.
When the control unit 5 confirms, on the basis of the detected currents supplied from the current transformers 4a and 4b, that the conducting currents of the thyristor switches 1U and 1W (i.e., currents flowing through the thyristor switches 1U and 1W, respectively, in the conducting states thereof) are zero, the control unit 5 first closes the vacuum electromagnetic contactors 2U, 2V and 2W and then turns on the thyristor switches 1U, 1V and 1W by applying a turn-on signal to the gates thereof in the state in which the vacuum electromagnetic contactors 2U, 2V and 2W are closed. Subsequently, the turn-on signal is cleared from the gates of the thyristor switches 1U, 1V and 1W to thereby allow them to be turned off. Then, the control unit 5 again checks and confirms on the basis of the detected currents supplied from the current transformers 4a and 4b that the conducting currents of the thyristor switches 1U and 1W again assume zero to thereby open the vacuum electromagnetic contactors 2U, 2V and 2W. This operation is what is referred to as the interlock operation which is performed with a view to elongating use life of the vacuum electromagnetic contactor.
Upon occurrence of a fault such as a short circuit event, the circuit breaking operation must of course be performed. In that case, the short-circuit fault is detected by the control unit 5 on the basis of a signal indicating the fault currents (e.g. short-circuit current) as supplied from the current transformers 4a and 4b to thereby remove the turn-on signal from the gate of the thyristor switches within 1 msec., as a result of which the fault current is interrupted by opening or turning off the thyristor switches 1U, 1V and 1W within one cycle (20 msec. at maximum).
With the arrangement of the conventional circuit breaker described above, the fault current such as short-circuit current may flow to the distribution system during a period corresponding to one cycle (e.g. 20 msec.). Consequently, when the short-circuit impedance of the transmission system is lowered due to change or alternation of the transmission system configuration, the value of the fault current increases. To cope with such situation, it is necessary to increase the short-circuit current breaking capacity of the circuit breaker provided at power receiving nodes or locations of power demanding systems or customer's equipment, which in turn means that the power demanding equipment has to be changed or modified correspondingly. Besides, the conventional circuit breaker suffers a problem that loss of the power in the conducting state is large when compared with that of the conventional mechanical type circuit breaker because the power supply is ordinarily performed through the thyristor switches.